Letter from mountain hospital on source of antidote (5)
I believe the answer is:
alpha
'letter' is the definition.
(alpha is a kind of letter)
'mountain hospital on source of antidote' is the wordplay.
'mountain' becomes 'alp' (alp is a kind of mountain**).
'hospital' becomes 'h'.
'on' is a charade indicator (letters next to each other).
'source of' indicates taking the first letters.
The first letter of 'antidote' is 'a'.
'alp'+'h'+'a'='ALPHA'
'from' acts as a link.
(Other definitions for alpha that I've seen before include "Most prominent star" , "Early testing stage for new software or hardware" , "Leading Greek character" , "Dominant" , "First-rate" .)